The field of this invention is that of the crossing of subsea pipelines and the damage which can happen to them when crossed. The pipeline infrastructure in the Gulf of Mexico (and other places in the world) on a map looks a lot like the road system on land. There are many pipelines, many pipeline intersections, and many pipelines which cross over one another.
The first problem of crossing pipelines is simply than one pipelines on the other can damage the coating on the pipelines and the difference in electrical potential can cause rapid galvanic corrosion on one or other of the pipelines. This problem is conventionally resolved by placing sandbags over the lower pipeline and laying the upper pipeline on the top of the pipelines.
The process of placing sandbags in subsea situations is expensive, and gets more expensive as depths increase.
After the sandbags are in place, if currents increase or the seafloor shifts enough to move the pipelines, the upper pipeline can slide to a location other than where the sandbags are. In the short time this scrapes the coatings off and promotes galvanic corrosion. In the longer term, the sandbags must be moved or replaced at repeated expense.